A study of the hypothesis that Attention Deficit Hyperacticity Disorder (ADHD) is a genetically influenced brain disorder will be undertaken through a genome wide search for loci linked to ADHD. To this end, affected sibling pairs and densely affected multi-generation Hispanic families will be recruited. Participating individuals will undergo a battery of psychological tests and will have blood drawn for the linkage analysis and positional cloning studies that will be used to search for genes associated with ADHD. To date, we have complete information on 5 large, multi-generation Hispanic families, and simulaton studies on 27 informative pedigrees indicate excellent power to detect linkage.